A NEW APPROACH TO QUANTIFY CENTIMETER-SCALE SURFACE ROUGHNESS USING THERMAL INFRARED ORBITAL DATA

New Routine Off-nadir Targeted Observations (ROTO) of the Thermal Emission Imaging Spectrometer (THEMIS) are used to validate prior surface roughness models and derive cm-scale surface roughness of Mars. This work examines negatively sloped thermal infrared (TIR) emission spectra resulting from surface anisothermality below the pixel-scale. Orbital TIR spectral and temperature data are shown to be excellent discriminators of surface physical properties and composition, the latter requiring accurate emissivity retrieval. However, increases in the sub-pixel abundance of cooler shadows and warmer sun-facing facets of rough surfaces result in negatively sloped emissivity spectra result, inhibiting accurate compositional analysis. Sloped emission spectra result from incorrect assumptions of a uniform pixel-integrated temperature or the maximum emissivity during temperature-emissivity separation of the radiance data. Spectral slopes are distinct, with magnitudes proportional to the degree of roughness. Off-nadir data are used here to separate these effects.

A series of eight ROTO datasets with emission angles ranging from -31° to +33° were acquired of Apollinaris Mons. ROTO viewing parameters include local true solar times from 18:00h -19:00h, solar longitudes (L_s) from 38° to 47°, and incidence angles from 94° to 102°. Negatively sloped emission spectra of various magnitudes were identified in all eight ROTO observations. By combining thermophysical modeling with a modified version of a prior surface roughness model, simulated emission data at varying RMS slope values are produced to match the observed TIR spectral slopes. Results indicate an average RMS slope of 4.7° at the 10 cm scale. Similar magnitude RMS slope values are measured in Death Valley, CA of alluvial fans composed of gravel to cobble sized clasts with a sandy matrix. HiRISE images taken of the study region show a fine-grained smooth surface, interspersed with boulders and the presence of aeolian ripples. Maximizing emission angle observations with variable look angles provides a more complete view of surface features, with higher emission angles exhibiting more thermal mixing than those viewed at nadir. ROTO data combined with thermophysical modeling allow for the first estimates of cm-scale RMS slope and Θ-bar roughness.